Piezoelectric materials comprising ferroelectric ceramics have heretofore been used in ceramic filters, mechanical filters, ultrasonic transducer and piezoelectric buzzers. When piezoelectric materials are used in such applications, resonance characteristics of the materials are utilized, and in this respect ferroelectric ceramics of PbTiO.sub.3 --PbZrO.sub.3 series optionally having BaTiO.sub.3, SrTiO.sub.3 and/or CaTiO.sub.3 added thereto have been generally used. Recently, to improved various characteristics of the ferroelectric ceramics of PbTiO.sub.3 --PbZrO.sub.3 series, ternary piezoelectric ceramics represented by PbTiO.sub.3 --PbZrO.sub.3 --Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 have also been proposed.
On the other hand, use of piezoelectric in the non-resonance state, for example use of piezoelectric ceramics as an actuator, has also been studied. However, when a piezoelectric ceramic material is used in the non-resonance state, it is necessary to transform electric energy to mechanical energy which displaces the piezoelectric ceramics material itself. Accordingly, piezoelectric ceramics having a large piezoelectric strain constant d are required.
Among the piezoelectric strain constant d, electromechanical coupling factor k and relative dielectric constant .epsilon., of a piezoelectric ceramic material, there is a following relation: ##EQU1## and therefore, in order that the material has a large piezoelectric strain constant d, it must has a large electromechanical coupling factor k and/or a large relative dielectric constant.
It is known in the art that piezoelectric ceramics of PbTiO.sub.3 --PbZrO.sub.3 --Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 --SrTiO.sub.3 series have a large relative dielectric constant and a large electromechanical coupling factor.